Problems with In-Building Communication are described in the Phoenix Fire Department Radio System Safety Project, Final Report, Version 1.7, dated Oct. 8, 2004, which is hereby expressly incorporated by reference and in the National Public Safety Telecommunications Council, NPSTC, Best Practices for In-Building Communications, dated Nov. 12, 2007, which is hereby expressly incorporated by reference.
During an emergency, fire department communications depend on land mobile radio systems. Land mobile radio systems are allowed to operate in portions of the radio spectrum under rules administered by the FCC. Portions of the spectrum are divided into bands where land mobile radio systems operate with frequencies in the 30 MHz (VHF low), 150 MHz (VHF high), 450 MHz (UHF), 700 MHz, and 800 MHz bands. Fire department communication systems may also use 154,280 (MHz) as this frequency is designated by the FCC as a mutual-aid radio channel. The bandwidth of radio channels, the amount of radio spectrum used by the signal transmitted by the radio, is set by the FCC to include a maximum and a minimum bandwidth for channels in each frequency band. A frequency number indicates the center of each radio channel with half of the bandwidth located on each side of the center.
Based on the limited radio spectrum, permitted bandwidth has been decreasing. Previously, channel bandwidths were 25 kHz. Newer rules require reducing bandwidths to 12.5 kHz. As the bandwidth has become more crowded, there is increasing difficulty to ensure quality reception. The reduced bandwidth means reduced energy to carry the same amount of information over the same distance increasing the challenge of reliable communications. The 800 MHz band is typically considered to have reduced naturally occurring interference. However, the large number of cellular phone company and other spectrum users has led to increasing man-made interference in the 800 MHz band.
During land mobile radio communications, a radio signal is sent from a transmitter to a receiver starting when the transmitter generates electromagnetic energy. This electromagnetic energy is converted by an antenna into electromagnetic waves. One type of electromagnetic radiation used in communication is commonly referred to as radio waves or radio signals. Radio waves sent outward from the antenna of the transmitter to the antenna of a receiver are considered downlink transmissions. The antenna of the receiver converts the electromagnetic waves back into energy, which is directed through a transmission line to a receiver. Radio waves then sent from the receiver to the original transmitter are considered uplink transmissions. When both the uplink and downlink transmissions are transmitted on the same frequency, it is considered simplex communication. Typically, one radio user may communicate directly with another radio user using simplex communications. In duplex mode of operation, when a radio user transmits a message, the message is received by a tower, which then retransmits the radio signal to other portable radio users. In half duplex operation or repeated radio communication, two radio frequencies are used for communication. The transmitting radio transmits on a first frequency to a repeater. The repeater then repeats the transmission on a frequency 2 and the signal is received by the receiving radio. Line-of-Sight (LOS) describes an unobstructed free-space link from a source antenna to a receiving antenna.
Variations in the signal level at the receiver may be due to manmade sources or natural structures including multipath interference, internal noise in the electronic circuit, structures blocking or obstructing pathway of the radio waves, natural noise, near-far interference, intermodulation interference, receiver desensitization interference, or receiver overload.
Typically, one of the most important factors for effective radio coverage is the location and orientation of the antenna to provide a direct path between the transmitter and the receiver. Important properties of the antenna include operating frequency, polarization, and radiation pattern. Optimizing these properties of the antenna in the desired range may provide improved radio coverage in desired areas and reduce interference in undesired directions. In particular, a directional antenna such as a Yagi antenna or a panel antenna may be used to provide a signal with increased gain near the front of the antenna and a weaker signal from the back.
In typical fire service operation, portable handheld radios powered with rechargeable or replaceable battery packs are used to communicate with base station radios, which may be powered by AC utility power. Mobile radios are designed for use in vehicles and may be powered from the vehicle's electrical system. Repeaters are capable of transmitting and receiving signals at the same time and can be used to extend the coverage of portable or mobile radios. Analog radios use frequency modulation to transmit a signal directly correlated to the microphone audio. Digital radios may also be used which have better spectrum efficiency than analog radios and have increased radio reception range in weak signal conditions relative to analog radios.
The Association of Public Safety Communications Officers (APCO) developed P25 as the standard in digital radio communications in preparation for the move to digital technology. While P25 was intended to serve as a common digital language for the radios and system infrastructures, system manufacturers developed proprietary features resulting in a loss of interoperability. Further, due to difficulty of digital radios to distinguish between high background noise levels and spoken voice data has led to P25 digital portable radios not being recommended for fire-fighting applications when an SCBA facepiece is being used. Analog modulation is preferred for situations where an SCBA (Self Contained Breathing Apparatus) facepiece is used, while law enforcement operations and emergency medical incidents and support functions by the fire department are likely to utilize digital radio technology. As a result of these different functional needs and preferences among the different personnel arriving at the scene of an emergency, different choices in radio equipment has been made increasing concerns over interoperability between different agencies using different models and radios from different manufacturers at the scene of a single event.
Free-space loss (FSL) is the loss in signal strength of an electromagnetic wave that results from a line-of-sight path through free space, which assumes no obstacles in the path to cause reflection or diffraction. The FSL is proportional to the square of the distance between the transmitter and receiver and also inversely proportional to the square of the wavelength and proportional to the square of the frequency. Other factors like the gain of antennas used by the transmitter or receiver or the loss associated with hardware imperfections are typically not considered in the FSL calculation.
The equation for FSL is as follows:FSL=(4πd/λ)2 FSL=(4πdf/c)2 where λ is the signal wavelength in meters;
f is the signal frequency in hertz;
d is the distance from the transmitter; and
c is the speed of light in a vacuum, 2.99792458×108 meters per second.
FSL may also be expressed in terms of decibel (dB).FSL(dB)=10 log10(((4π/c)df)2))In radio applications, using f, in units of MHz and d in units of km provides the following relationship.FSL(dB)=20 log10(d)+20 log10(f)+32.45
As the electromagnetic energy in the radio wave spreads out over free space there is a reduction in power of the signal. This is shown by the following equation.S=Pt(1/(4πd2))where:    S is the power per unit area or power spatial density (in Watts per meter squared) at distance, d, and Pt is the total power transmitted (in Watts).
The receiving antenna's aperture is a measure of how well an antenna can pick up power from an incoming electromagnetic wave. This relationship for an isotropic antenna is shown in the following equation:Pr=Sλ2/4π    where Pr, is the received power. The total loss is given by the following equation:FSL=Pt/Pr 
Additionally, as the radio wave travels from the transmitter to the receiver, different paths traveled by the electromagnetic waves before reaching the receiver may affect the signal quality. Objects in the path of the radio waves such as buildings, trees, or local terrain will reduce the strength of the waves by absorbing or interrupting the signal. The density, size, shape, and type of material obstructing the path of the waves will determine if the waves are reduced, blocked, absorbed, or reflected before reaching the receiver. The final engineered system should consider all the gains and losses in a specific structure to provide a more realistic expectation of coverage according to the following equation:RxP=TxP+TxG−TxL−FSL−ML+RxG−RxLwhere:
RxP=received power in dBm;
TxP=transmitter output power in dBm;
TxG=transmitter antenna gain in dBi;
TxL=transmitter losses (coax, connectors . . . ) in dB;
FSL=free space loss or path loss in dB;
ML=miscellaneous losses (fading, body loss, polarization mismatch, other losses);
RxG=receiver antenna gain in dBi;
RxL=receiver losses (coax, connectors) in dB.
In order for a radio user to communicate when they are in a building or other structure, the radio waves must be strong enough after traveling through space to penetrate the structure of the building. Increased distance from the communications tower where the radio waves are generated can lead to weakened radio signals making it difficult for the radio waves to provide coverage inside a building. In-building coverage level (the coverage of a radio system in the interior of a building) is affected by the type of materials used in the construction of the building as well as the distance from the radio tower. Generally, the heavier the construction materials, the higher the dB level needed for the radio waves to penetrate into the structure to provide in-building communication.
The National Public Safety Telecommunications Council (NPSTC) issued a Best Practices for In-Building Communications publication on Nov. 12, 2007 to provide reliable communications methods inside buildings, basements, stadiums, and tunnels. In this publication, the three primary methods for attaining In-Building Communication are: 1) utilizing additional antenna sites within a jurisdiction to increase signal level; 2) supplementing coverage in a specific building with a permanent system to boost the signal received and boost the signal transmitted to the outside; and 3) deploying a system on a temporary basis to boost coverage in a building for a specific incident scene.
Increasing signal strength with additional antenna sites is typically limited by the cost, spectrum availability, approval, as well as structural obstacles preventing certain building structures from receiving adequate signal necessary for In-Building Communication. Supplemental coverage in specific buildings may add additional expense for treating each building separately and these individual systems may create problematic interference. Most importantly, permanent building specific solutions may not be reliable during an actual event at the building location. For example, a building fire may damage these systems or the antenna or power lines essential for communications.
Deployable communications systems provide a practical approach to improving radio coverage and backup existing systems during an incident. Bi-directional amplifier communications systems are subject to oscillation when there is inadequate isolation (path loss) between the transmitting and receiving antenna. Because this type of oscillation can lead to serious interference disrupting other communications in the nearby area, it is illegal to operate a signal booster that oscillates and the FCC (FCC) may impose fines and confiscate equipment.
According to CFR 47 Section 90.7 of the FCC, a signal booster is “a device at fixed location which automatically receives, amplifies, and retransmits on a one-way or two-way basis, the signals received from base, fixed, mobile, and portable stations, with no change in frequency or authorized bandwidth. A signal booster may be either narrow band (Class A), in which case the booster amplifies only those discrete frequencies intended to be retransmitted, or broadband (Class B), in which case all signals within the passband of the signal booster are amplified.”
Under this designation, class A signal boosters are considered to be channelized amplifiers.
An RF amplifier that is able to select what frequencies are to be amplified in the downlink and uplink paths and increases the RF signal strength in both directions is known as a bi-directional amplifier.
Typically, the desired signal strength delivered to the facility is at least −95 dBm through at least 95 percent of the facility in non-critical areas and 99% of the facility in critical areas such as fire control rooms and exit corridors. The in-building environment should be isolated from the outside of the building to prevent detrimental oscillations. Typically, 15 dB more than the gain of the deployable communication system booster is an appropriate amount of isolation between the two inside and outside. For a 90 dB gain booster/BDA, the ideal isolation situation would be at least 105 dB of isolation for example.
While the FSL is governed by the equations listed above when the wave travels through free space, when the wave encounters a solid object such as a building wall the wave can be further weakened significantly. A radio wave may lose as much as 40 dB or more in signal strength when passing through the wall of a building. Transmission of radio signals through wire or cable must also be evaluated independent of FSL. Generally, radio signal strength losses at 800 MHz frequencies, typically used in public safety radio systems, may be about 4 dB or more per approximately 100 feet of low loss type coaxial cable. The losses realized by sending radio waves through a wall via a low loss cable may be advantageous compared to the loss attributable to building wall attenuation realized when sending radio waves through thick walls as an entirely wireless transmission, especially when the former is combined with an amplifier system.
U.S. Pat. No. 4,476,574 to Struven describes a method of providing multiple channels of mobile-to-mobile radio communication in tunnels, mines, buildings, and other confined spaces using radiating transmission lines.
U.S. Pat. No. 4,905,302 to Childress et al. describes a method for using a trunked radio repeater system in a public service trunked (PST) system and special mobile radio (SMR) application.
U.S. Pat. No. 6,032,020 to Cook et al relates to the operation of multiple repeaters utilizing a single communication infrastructure as a fixture within a building to provide communications past a barrier.
US Pat. Pub. No. 20060148468 to Mann relates to the field of in-building radio communication coverage enhancement utilizing a primary external antenna, an ancillary external antenna, a donor site diversity system, an internal antenna, and a bi-directional amplifier.
US Pat. Pub. No. 20070099667 to Graham discloses an in-building wireless enhancement system for high-rises with an emergency backup mode of operation including a wireless base station, a backbone coupled to the base station, a plurality of coupler units connected to the backbone, a first plurality of antennas, a plurality of amplifiers connected to the backbone, a second plurality of antennas, and optionally an emergency access port coupled to the backbone.